The third phrase of the text is sung to this musical phrase:
In the first verse, the text here is "For Your mercies never ending" (which I think should be "never-ending"), and in the second verse, the text is "As we tend that endless treasure." Both "never-ending" and "endless treasure" are sung with melismas (the string of eighth notes in the second measure), and because these words are stretched over these extra syllables, there's a musical sense of that extended period of time.
Similarly, there's a melisma for "unnumbered" in the refrain ("Lord, we pray that we, Your people / Who Your gifts unnumbered claim..."). Here's the music for just the second line:
Because "unnumbered" is sung with an extra syllable, there's a musical sense of the abundance of God's gifts.